Laser Interference Lithography Research Articles

Effects of azimuthal angles on laser interference lithography.

This paper discusses the effects of azimuthal angles on two-, three-, and four-beam laser interference. In two- or three-beam laser interference, periodic surface structures of lines or dots were obtained. In four-beam laser interference with the polarization mode of TE-TM-TE-TM, the modulation in a particular direction was formed and calculated. In the work, a He-Ne laser system was used to simulate two-, three-, and four-beam laser interference, and the interference pattern was detected by a CCD. A high-power Nd:YAG laser interference lithography system was set up to pattern silicon wafers. In the experiments, one azimuthal angle was changed every time to form interference patterns when polarization states were fixed and incident angles were equal. The experimental results have shown that the azimuthal angle affects the periods and feature sizes of the interference patterns and the fabricated surface structures, which are in accordance with the theoretical and computer simulation results.

Nanopost-guided self-organization of dendritic inorganic salt structures.

The formation of hierarchical architectures is of fundamental importance and yet a relatively elusive problem concerning many natural and industrial processes. In this paper, a nanopost array platform, or a nanopost substrate, has been developed to address this issue through a model study of the drying structures of phosphate buffered saline (PBS) solution. Unlike on a plain surface, highly ramified salt structures are formed by simply allowing the nanopost substrate wetted with the salt solution to dry, a process that completes within minutes at room temperature. The branches of salt structures have similar shapes repeating at different length scales, ranging from ∼200 nm up to a few centimeters in length, covering a 2 × 2 cm(2) area patterned with nanoposts fabricated in photoresist via laser interference lithography (LIL). Scanning electromicrograph (SEM) images show that salt structures are formed around nanoposts, and characteristic features of these salt structures can be modulated and predicted based on the surface properties and geometrical arrangements of nanoposts, suggesting that nanoposts can be used to guide the organization and crystallization of salts. This nanopost-guided crystallization approach is robust, rapid, versatile, and amenable to real-time observation and mass production, providing a great opportunity for the study and creation of large-scale hierarchical structures.

Nanoshuttered OLEDs: Unveiled Invisible Auxiliary Electrode

In this study, organic light‐emitting diodes (OLEDs) with enhanced optical properties are fabricated by inserting a nanosized stripe auxiliary electrode layer (nSAEL) between the substrate and an indium tin oxide (ITO) layer. This design can avoid the shortcomings of conventional microsized layers while maintaining high optical uniformity due to the improved conductivity of the electrode. The primary advantage is that the nSAEL (submicrometer scale) is no longer visible to the naked eye. Moreover, the reflective shuttered (grating) structure of the nSAEL increases the forward‐directed light by the microcavity (MC) effect and minimizes the loss of light by the extracting the surface plasmon polariton (SPP) mode. In this study, the degree of the MC and SPP can be controlled with the parameters of the nSAEL by simply conjugating the conditions of laser interference lithography (LIL). Therefore, the current and power efficiencies of the device with an nSAEL with optimized parameters are 1.17 and 1.23 times higher than the reference device at 1000 cd/m2, respectively, and at these parameters, the overall sheet resistance is reduced to less than half (48%). All of the processes are verified by comparing the computational simulation results and the experimental results obtained with the actual fabricated device.

Electronic-controlling nanotribological behavior of textured silicon surfaces fabricated by laser interference lithography

The electronic-controlling nanotribological behavior of textured silicon surfaces was investigated by an atomic force microscope (AFM), which were fabricated by laser interference lithography and chemical etching. The results indicated that without an external bias, friction on textured surfaces was greater than that on smooth surfaces and it was related to the coverage rate of the microgrooves. With an external bias, friction on smooth and textured surfaces decreased at first and then increased approximately linearly as the external bias increased.

Fabrication of Periodic Microstructures on Nickel Template by Multi-Beam Laser Interference

Complex microstructures can be fabricated on nickel template directly by femtosecond laser. But the efficiency is low and much time is inevitable when large scale microstructures are fabricated. The multi-beam interference provides an effective method for fabrication of large scale microstructures. This paper mainly focuses on direct ablation of nickel template by four-beam interference and fabricating periodic microstructures by laser interference lithography. Experimental results show that submicron periodic stripes and column structures can be produced on the nickel template by laser interference, laying the foundation of making nickel molds for microfluidic chips.

Large‐Area Antenna‐Assisted SEIRA Substrates by Laser Interference Lithography

Laser interference lithography is utilized to fabricate large‐area plasmonic antenna substrates for surface‐enhanced infrared absorption (SEIRA). Changing the interference condition in each exposure process allows precise control over the geometrical parameters of the structures and thus tailoring of their optical response. Independent of the underlying wafer, the technique enables a homogeneous preparation of antennas over cm2 areas with tunable and high‐quality plasmonic resonances in the near‐ and mid‐infrared spectral range. The broad applicability of such SEIRA substrates is demonstrated by enhancing the infrared vibrational signals of a monolayer octadecanethiol and by monitoring the ultraviolet degradation of a polymer via the decrease of its specific vibrational modes. Due to the large‐area and fast fabrication process, laser interference lithography is ideally suited for the preparation of low‐cost sensing platforms for a variety of different SEIRA applications in the chemical, analytical, and life sciences.

Palladium-decorated hydrogen-gas sensors using periodically aligned graphene nanoribbons.

Polymer residue-free graphene nanoribbons (GNRs) of 200 nm width at 1 μm pitch were periodically generated in an area of 1 cm(2) via laser interference lithography using a chromium interlayer prior to photoresist coating. High-quality GNRs were evidenced by atomic force microscopy, micro-Raman spectroscopy, and X-ray photoelectron spectroscopy measurements. Palladium nanoparticles were then deposited on the GNRs as catalysts for sensing hydrogen gases, and the GNR array was utilized as an electrically conductive path with less electrical noise. The palladium-decorated GNR array exhibited a rectangular sensing curve with unprecedented rapid response and recovery properties: 90% response within 60 s at 1000 ppm and 80% recovery within 90 s in nitrogen ambient. In addition, reliable and repeatable sensing behaviors were revealed when the array was exposed to various gas concentrations even at 30 ppm.

Micro- and submicrostructuring thin polymer films with two and three-beam single pulse laser interference lithography.

In this work we report the application of two and three-beam single pulse laser interference lithography to thin polymer films of poly(trimethylene terephthalate) (PTT). By irradiating the sample surface with temporary and spatially overlapped single pulses from two or three coherent beams and changing the angles of incidence, we have accomplished the fabrication of large-area polymer micro and submicrogratings as well as submicrometric cavities arranged in a hexagonal lattice. The characterization of the structures in real space by atomic force microscopy (AFM) and scanning electron microscopy (SEM) has allowed us to determine the formation mechanism of the microgratings to be based on different ablation regimes depending on the local fluence. Moreover, complementary characterization of the submicrometric cavities in reciprocal space by grazing incidence small-angle X-ray scattering (GISAXS) confirms the existence of large areas where two-dimensional order is present. The experiments presented in this work demonstrate the suitability of single pulse laser interference lithography for micro and submicrostructuring polymer films, opening up new possibilities for patterning and paving the way for potential applications where polymer structures are involved.

Design and fabrication of broadband ultralow reflectivity black Si surfaces by laser micro/nanoprocessing

Light collection efficiency is an important factor that affects the performance of many optical and optoelectronic devices. In these devices, the high reflectivity of interfaces can hinder efficient light collection. To minimize unwanted reflection, anti-reflection surfaces can be fabricated by micro/nanopatterning. In this paper, we investigate the fabrication of broadband anti-reflection Si surfaces by laser micro/nanoprocessing. Laser direct writing is applied to create microstructures on Si surfaces that reduce light reflection by light trapping. In addition, laser interference lithography and metal assisted chemical etching are adopted to fabricate the Si nanowire arrays. The anti-reflection performance is greatly improved by the high aspect ratio subwavelength structures, which create gradients of refractive index from the ambient air to the substrate. Furthermore, by decoration of the Si nanowires with metallic nanoparticles, surface plasmon resonance can be used to further control the broadband reflections, reducing the reflection to below 1.0% across from 300 to 1200 nm. An average reflection of 0.8% is achieved. Using lasers to process silicon is a convenient way of creating textured surfaces that exhibit broadband anti-reflection properties. Yang and co-workers from Singapore and China used laser interference lithography and etching to create arrays of high aspect ratio, thin silicon nanowires, which they then decorated with metallic nanoparticles. The researchers say that the low reflectivity of their ‘black silicon’ is due to the nanowires' light-trapping properties and low refractive index, combined with surface plasmon resonances associated with the nanoparticles. Measurements indicate that the reflectivity of the treated surface was less than 1% across the ultraviolet, visible and near-infrared spectral regions (300–1200 nm). Because laser processing can pattern surfaces rapidly, the researchers believe that their scheme could be a cost-effective way of manufacturing large-area broadband anti-reflection coatings.

Ultraviolet pulsed laser interference lithography and application of periodic structured Ag-nanoparticle films for surface-enhanced Raman spectroscopy

Ag-nanoparticle films were deposited on glass and silicon substrates by evaporation of a colloidal solution. Periodic structures were created in these Ag nanoparticle deposits by ultraviolet nanosecond laser interference lithography (LIL). We find that the periodic structure and size of Ag nanoparticles in these oriented arrays can be controlled by varying laser pulse energy as well as the number of pulses used in irradiation. The effect of these parameters on the overall morphology of these deposits is shown to be promising as a new technique for the engineering of sub-micron nanoparticle structures. Ag films with laser-induced sub-micron periodic structures were evaluated as substrates for surface-enhanced Raman spectroscopy using rhodamine B adsorbed from a 10−8 M solution. Enhancement factors were found to be >106 those found in conventional Raman scattering and 10 times greater than those from standard Ag-nanoparticle films. These results indicate that the LIL method has promise in the nanostructuring of Ag-nanoparticle films for ultrasensitive SERS biosensing substrates.

P‐146: Enhancing Light Extraction with a Reduced Spectrum Distortion of Organic Light‐Emitting Diodes by Employing Nano‐Scale Periodic Structure

AbstractIn this study, we demonstrate high‐efficiency distortion‐free spectrum Organic Light‐Emitting Diodes (OLEDs) using photonic crystal (PC) and micro lens array (MLA). The PC structure is fabricated by laser interference lithography (LIL), which enables simple and cost‐intensive manufacturing for mask less and largely scalable process. Finally, we obtain a significant results that external quantum efficiency (EQE) and the power efficiency (PE) were enhanced by up to 150% and 250%, as compared reference.

P‐151: Optical Efficiency Enhancement of Organic Light‐Emitting Diode Based on a Nano‐Sized Stripe Auxiliary Electrode

AbstractWe fabricated the organic light‐emitting diodes (OLEDs) having enhanced optical efficiency with nano‐sized stripe auxiliary electrode. It could supplement the shortcomings of micro‐sized that while maintaining the high optical advantages. We can simply deal with the parameters of nano‐sized auxiliary electrode by conjugating the conditions of laser interference lithography (LIL). The demonstrated process can be also easily acceptable to mass production of the large area OLEDs applications.

Throughput comparison of multiexposure and multibeam laser interference lithography on nanopatterned sapphire substrate process

In this research, we describe a new approach, i.e., laser interference lithography (LIL), to fabricating pattern sapphire substrate (PSS) and nanopatterned sapphire substrate (NPSS). There are two ways to pattern hexagonal 2D arrays on sapphire by LIL: multiexposure LIL and multibeam LIL. In conclusion, the simulation results in this research verified that multibeam LIL has significantly better throughput than multiexposure LIL. The result also supports the advantages of a negative photoresist over a positive photoresist.

Fabrication of moth-eye structures on silicon by direct six-beam laser interference lithography

This paper presents a new method for the generation of cross-scale laser interference patterns and the fabrication of moth-eye structures on silicon. In the method, moth-eye structures were produced on a surface of silicon wafer using direct six-beam laser interference lithography to improve the antireflection performance of the material surface. The periodic dot arrays of the moth-eye structures were formed due to the ablation of the irradiance distribution of interference patterns on the wafer surface. The shape, size, and distribution of the moth-eye structures can be adjusted by controlling the wavelength, incidence angles, and exposure doses in a direct six-beam laser interference lithography setup. The theoretical and experimental results have shown that direct six-beam laser interference lithography can provide a way to fabricate cross-scale moth-eye structures for antireflection applications.

Simulation and fabrication of nanostructures with laser interference lithography

This article presents laser interference lithography, a preferred method to achieve periodic structures in a large area with fast speed and low cost. Simulations and experiments for two-beam interference lithography with single exposure and double exposure were expressed. In the experiments, patterns on photoresist were transferred to a silicon substrate by wet etching. The experimental results were consistent with the simulative results. The one-dimensional nanogrooves were obtained by single exposure, and different widths of nanogrooves during the same period were fabricated on silicon substrates by varying exposure time. In addition, two-dimensional grid patterns were obtained by double exposure. With the increasing of exposure time, the grid pattern changed from a nanoholes array to a nanodots array. The results showed that laser interference lithography is an efficient method to fabricate nanostructures with various geometrical parameters by controlling exposure time, which has a promising application in the study of nanotextured surface properties, such as wetting, adhesion and friction.

Near-Field Hot Spots in Gold Nanoplasmonic Templates and Their Use for Surface Enhanced Raman Scattering Sensing Application

Finite difference time domain (FDTD) method is adapted to investigate near-field enhancement effects on plasmonic structures (patterned in gold film) such as concentric rings with small separation, square, and rectangle. The near-fields effect on surface enhanced Raman scattering (SERS) is typically studied on square and rectangular structures. These metal structures are fabricated by laser interference lithography. Raman active molecules (Rhodamine 6G in PMMA (polymethyl methacrylate)) are spread onto patterned structure by spin coating, and Renishaw inVia Raman spectrometer was used to study SERS. Typical SERS enhancement of the order of 105 is seen for square and rectangular structures. It is observed that the corner points and edges of square and rectangular structures are most sensitive to concentrate near fields. In the case of concentric rings, huge near fields are seen to exist at the gap between the metal rings. Concentric rings are proposed to be very effective structure for SERS sensing applications such as molecular identification and biological mapping.

Large area plasmonic nanoparticle arrays with well-defined size and shape

We demonstrate an innovative process to fabricate uniformly shaped plasmonic nanoparticles. Laser interference lithography, nano-imprint lithography and a lift-off process are employed for the controlled production of periodically arranged nanoparticles on large areas. Round and elliptic silver particles with diameters of about 200 nm on an area of 5×5cm2 are investigated. Measurements of resonant absorption by the metal particles are in agreement with data computer-simulated by rigorous coupled wave analysis. We observe that the plasmonic resonance of elliptic particles depends on the polarization of incident light and that porosity of the metal influences the plasmonic band.

Sensors Research at the NSF-Assist Nanosystems Engineering Research Center:Correlated Sensing of Environmental and Physiological Parameters Using Low-Power Wearable Sensors

This presentation will provide an overview of the technology and system development effort for low-power wearable sensors in the NSF Nanosystems Engineering Research Center (NERC) for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST). We employ nanotechnology as an enabling factor to achieve low-power, highly specific, multifunctional sensing on compact wearable platforms. Minimizing the power consumption by the sensors and supporting electronic circuits is a critical goal, as these sensors would be powered by harvested energy. To achieve these goals we develop a systems-based approach that optimizes the overall system performance instead of focusing solely on individual components. We envision that nanotechnology-enabled long-term sensing will enable patients, physicians, and scientists to make direct correlations between health and environmental toxins leading to chronic disease prediction, management, and treatment. Furthermore, the self-powered, wearable, correlated sensing platforms we develop will accelerate environmental health research and clinical trials as well as inform environmental policy.Based on the targeted sensing parameter, ASSIST sensors can be grouped in two categories: 1) Environmental sensors (i.e., for sensing pollutant gases and particulate matter); 2) Physiological sensors (i.e., for sensing heart rate, hormonal biomarkers, oxygen saturation, body sounds, and hydration levels). An alternative classification is based on the sensing mechanism: 1) Chemical/particulate sensing (i.e., for sensing gases such as ozone, nitric oxide, nitrogen dioxide, hydrogen sulfide, volatile organic compounds, and particulate matter); 2) Biochemical sensing (i.e., for sensing hormonal biomarkers such as cortisol and epinephrine, as well as electrolytes); 3) Bioelectrical sensing (i.e., electrocardiogram and skin conductivity); 4) Optical and acoustic sensing (i.e., pulse oximetry and body sounds).For sensing the pollutant gases in the environment we develop four different types of sensor technologies: 1) Metal-oxide nanowires deterministically assembled on a custom low-power CMOS integrated circuit. Nanowires offer advantages such as low-power heating and increased surface-to-volume ratio for improved sensitivity. These nanowires are fabricated by coating silicon nanowires by a metal-oxide shell using atomic layer deposition (ALD). After these nanowires are released in a solution they are assembled on a CMOS chip. An alternative approach to make metal oxide nanowires is laser interference lithography. 2) An ALD metal-oxide layer by itself can also function as a gas sensor, which provides a large sensing surface area and low power heating. 3) Mechanically resonant devices with selective polymer coatings can sense gas. We use tuning fork type crystal resonators as well as electrostatically actuated flextensional transducers with nano-patterned selective polymers as functionalization layers. 4) AlGaN/GaN heterostructures to make open gate transistors with metal-oxide or nanoparticle functionalization layers. The high mobility in these heterostructures with specific functionalization layers has the potential to realize a low-power, highly sensitive chemical sensor. For sensing the particulate matter we develop an optical fiber based particle counting system that is suitable for micron-size particle detection. Our biochemical sensing strategy has three main components: 1) Sample extraction. We currently develop hydrogel-based microfluidic interfaces to skin to be able to sample sweat and interstitial fluids for further analysis. 2) Antibody-based functionalization for cortisol detection. 3) Transduction to an electrical signal by electrochemical impedance spectroscopy using nanowire enhanced interdigitated electrodes or tunneling field effect transistors. The major challenges for the described biochemical sensors are: 1) Noninvasive access to interstitial fluid; 2) Assuring antibody stability; 3) Achieving reversible binding of target analyte or finding a way to replenish antibodies for long-term usability of the sensor.The hydrogel-based skin interfaces are designed as multifunctional devices that can provide sweat sampling and facilitate direct measurement of skin conductivity by incorporating metal electrodes in the structure. Furthermore, pH sensing and other electrochemical sensing modalities can be integrated in this device.We have also implemented a low-profile wireless pulse oximetry system in the form of a smart bandage using commercial off-the-shelf components. Future generations of this device will include custom designed optoelectronic components for low-power operation.In summary, we are developing a wide variety of sensing modalities for continuous environmental and physiological monitoring. Low-power operation, compact and comfortable form factor, and multifunctionality are the common features in all of the described sensor systems. The technologies and systems we are developing in ASSIST have a great potential to improve global health by correlating personal health and personal environment. Acknowledgments: This material is based upon work supported by the National Science Foundation under Grant No. 1160483.

Superhydrophobic dual micro- and nanostructures fabricated by direct laser interference lithography

A method for the fabrication of highly ordered superhydrophobic dual micro- and nanostructures on silicon by direct laser interference lithography (LIL) is presented. The method offers its innovation that the superhydrophobic dual micro- and nanostructures can be fabricated directly by controlling the process of four-beam laser interference and the use of hydrofluoric acid (HF) to wipe off the silica generated during the process. Different laser fluences, exposure durations, and cleanout times have been investigated to obtain the optimum value of the contact angle (CA). The superhydrophobic surface with the CA of 153.2 deg was achieved after exposure of 60 s and immersion in HF with a concentration of 5% for 3 min. Compared with other approaches, it is a facile and efficient method with its significant feature for the macroscale fabrication of highly ordered superhydrophobic dual micro- and nanostructures on silicon.

Design and fabrication of birefringent nano-grating structure for circularly polarized light emission

Three different nano-grating structures are designed as phase retarders that can transform linearly polarized light to circularly polarized emission for the wavelengths of 488 nm, 532 nm and 632.8 nm, respectively. Gold based nano-grating structures with various periods are fabricated by utilizing laser interference lithography. The ellipticity of all circularly polarized emission can reach around 90% such that the structure has great potential in the applications of three-dimensional (3D) display. The effects of the slit width and metal thickness modulations are simulated by rigorous coupled wave analysis (RCWA) method. Besides, the field intensity and phase of the transmitted TM and TE waves are also simulated to understand their polarization characteristics.